The present invention pertains to an improved fibrous polyacrylonitrile mixture for use in the manufacture of friction products. The improved mixture is comprised of cut and refined polyacrylonitrile fibers in conjunction with an additive selected from the group consisting of:
(a) polyethylene glycol esters of pelargonic acid; or PA0 (b) polyethylene glycol esters of enanthic, caprylic or capric acids; or PA0 (c) blends of polyethylene glycol esters of enanthic, caprylic, pelargonic, or capric acids; or PA0 (d) blends of polyethylene glycol esters of carboxylic acids derived from natural products containing at least 50% by weight of carboxylic acids containing less than 14 carbon atoms; or PA0 (e) reaction products of ethylene oxide and carboxylic acid amides wherein at least 70% of the acids from which the amide is derived contain between 16 and 20 carbon atoms.
A decline in the use of asbestos in friction products has been occurring due to the resulting health hazards asbestos has created in both the workplace and the environment. Asbestos has provided uniquely favorable frictional characteristics and physical properties in brake and bearing products. Currently there is considerable research activity in an effort to find a suitable replacement for asbestos in friction product end uses. It is the object of the present invention to provide an asbestos substitute for the manufacture of friction products.
It has been found that of the synthetic fibers, acrylic fibers are favorably suited for use in friction products, as they do not melt as readily as nylon, polyester, polypropylene, etc. Furthermore, under appropriate conditions of heat and pressure, acrylic fibers are transformed into carbon fiber precursors and eventually into carbon fibers. These characteristics make acrylic fibers especially suited for use in friction products.
The manufacture of friction products (e.g. brake blocks) is carried out by placing a mixture into a first mold, and thereafter applying pressure to the mixture for a period of time in order to create a "preform". The preform is then removed from the first mold and placed into a second mold within which the preform is subjected to both heat and pressure. During the application of heat and pressure in the second mold, the preform is transformed into a friction product (e.g. a brake block).
In this manufacturing process, the preform must have a degree of integrity high enough so that it may be removed from the first mold and placed in the second mold without significant disintegration. All mixtures which are subjected to the preforming operation tend to "rebound" from the pressure exerted in the preforming operation. Theoretically, it is believed that the lower the degree of resilience, the better the quality of the preform. Too much resilience creates two detrimental consequences: (1) the degree of resilience is so high that the preform disintegrates upon the necessary handling required to remove the preform from the first mold and place it in the second mold; (2) the degree of resilience is so great that the preform expands by an amount so great that the preform will not fit into the second mold.
Although acrylic fibers have advantages (over many other synthetic fibers) in friction product applications, acrylic fibers having no liquid additives thereon have not been found to enable the production of satisfactory preforms. Furthermore, it has been found that in order to apply an operable amount of these liquid additives to the acrylic fibers, the fibers must be in a "wet gel" state. The structure of a wet gel is extremely "open" and absorbent (i.e. the polymer structure will hold large amounts of liquids, similar to a sponge) in comparison with a collapsed polymer structure. In addition, it has been found necessary to refine the wet gel fibers in order to create a fibrous pulp, rather than using continuous filaments, cut staple, or even comminuted fiber in the manufacture of friction products. Most preferably, the refining of the acrylic wet gel is performed in the liquid additive.
It has been found that liquid additives to the acrylic wet gel are taken up and thereafter held in amounts much greater than expected. For example, PEG - 400 monopelargonate has been applied to wet gel to a saturation point, after which the excess pelargonate was removed by centrifugation. The wet gel was then dried in a dryer, which caused the polymer structure to collapse. The resulting dried fiber was analyzed for PEG - 400 monopelargonate content, and it was found that the fiber contained over 50% (on weight of fiber) of PEG - 400 monopelargonate. This is extremely unusual as it was not believed previously that more than about 2% liquid (on weight of fiber) could be held by a dried acrylic fiber.
As described herein, the phrase "liquid additive" is intended to comprise not only additives which are themselves a liquid at room temperature and 1 atm. pressure, but also compounds which are liquefied, dispersed, or dissolved. It has been conceived that liquefied, dispersed, or dissolved additives within the group (a) through (e) above will be operable in enabling one to achieve the advantages of the present invention.
Furthermore, it has been unexpectedly found that certain liquid additives, when incorporated into the acrylic fiber in amounts greater than about 37% (on weight of fiber), dramatically decreased the resilience of the fiber as measured by a "white pellet test". This test is performed by placing a weighed 5 gram sample of dried, refined fiber having additive thereon into a press having a 1 inch (circular) cross-sectional area. The press is used to apply 5000 pounds of pressure to the 5 gram sample. The pressure was held for approximately 60 seconds. The thickness (i.e. height) of the pellet was measured while the pellet remained under pressure, this measurement being taken approximately 60 seconds after pressure was applied to the fiber. The fiber, having been pressed into a cylindrical "pellet" shape, remained in the press after pressure was removed, and the pellet was then removed from the press. The height of the pellet was then measured again approximately 5 minutes after removal of the pressure from the pellet. The percent rebound (i.e. resilience) was calculated as follows: